Midway through Steven Soderbergh’s latest film—the prolific filmmaker’s second after returning from a self-imposed retirement in 2013—there’s an unexpected cameo by a notable actor who’s worked with the director more than a half-dozen times. During this pivotal flashback, this Major Hollywood Star plays a Boston security consultant who tells the film’s heroine, Sawyer Valentini (Claire Foy), to “think of your cellphone as your enemy.” Certainly, that’s good advice for a woman who goes so far as to uproot from Beantown, leaving her friends and family behind to make a fresh start in Pennsylvania after two years of being terrorized by a stalker.

But for Soderbergh—who seems wholly re-energized based on this experimental “social thriller” and his first post-retirement picture, Logan Lucky—his smartphone is his best friend, at least on this project.

Unsane, Soderbergh’s down-and-dirty, low-budget production, was photographed almost entirely (save for some aerial drone shots) with three iPhones. And while it sounds like a gimmick, this decision lends an in-your-face immediacy to the picture, whose cold, ugly images feel like they’ve been captured by a fly-on-the-wall.

Of course, this voyeuristic approach is not entirely new for the director, who launched his career nearly three decades ago with Sex, Lies, and Videotape, a movie that helped put the Sundance Film Festival on the map. But what’s almost unheard of in this era of blockbuster filmmaking and ballooning budgets is that Unsane ended up costing almost exactly the same to produce as that 1989 drama. That’s why Soderbergh likes to joke that his entire camera department for this film fits into a backpack.

Beyond the technical aspects, however, the horrific goings-on are anchored by an unleashed performance from Foy, an actress best known for displaying regal restraint in her role as Queen Elizabeth II on Netflix’s The Crown. Although intellectually acute, Sawyer’s still emotionally unbalanced from her recent trauma, which leads her to be duped into unwittingly committing herself for 24 hours of psychiatric evaluation at the Highland Creek Behavioral Center after she consults with a staff therapist.

Locked inside close quarters with a group of previously committed patients such as unstable firebrand Violet (Juno Temple) and recovering opioid addict Nate (Jay Pharoah), Sawyer erupts into terror once she catches sight of a familiar face. Convinced that staff member George Shaw (Joshua Leonard) is actually David Strine, the stalker she moved more than 300 miles to escape from, she violently lashes out, which results in an extension of  her involuntary stay.

The script by writing partners Jonathan Bernstein and James Greer gradually paints Sawyer as an unreliable protagonist—not unlike Jack Nicholson’s R.P. McMurphy in Milos Forman’s One Flew Over the Cuckoo’s Nest—leaving us to wonder whether she’s actually being held against her will, trapped in a facility with a worker who’s a danger to her and others, or if she’s suffering from psychosis. Unfortunately, Bernstein and Greer tip their hand a bit too early, which dilutes the impact of the film’s final act.

Despite this, Soderbergh has a grand time staging the tale in exquisitely framed, personal space-invading close-ups and claustrophobia-inducing angles that are easily facilitated by the small size of his unconventional choice of camera. Rather than looking cheap, the iPhone-lensed images capture a certain queasiness, as though you’re witnessing something that you shouldn’t.

The film also benefits from the presence of Amy Irving, an actress who’s no stranger to thrillers, appearing in Brian De Palma’s Carrie (1976) and The Fury (1978) early in her career. Now, 18 years after acting in Soderbergh’s Oscar-winning Traffic, she reunites with him to play Angela Valentini, Sawyer’s concerned mother, who rushes to her daughter’s aid when the troubled girl can reach her with, yes, a cellphone.

Just have the courtesy to leave yours turned off as you sit in your seat, focusing not on the little screen in your pocket but on the big one before you, as Soderbergh masterfully adds another genre to his resume. And although the multiplex crowd will be titillated with tense chills, art house fans will be happy to know that the filmmaker hasn’t forgotten them, either. Beneath the film’s puzzle, he slyly examines the lengths to which institutions might go to make a profit—and that might just be the most chilling aspect of all. ◆

Unsane ★★★

Starring Claire Foy, Joshua Leonard, Jay Pharoah, Juno Temple, Colin Woodell, Myra Lucretia Taylor, Lynda Mauze, Zach Cherry, Polly McKie, Gibson Frazier, Aimee Mullins, Raul Castillo, Michael Mihm, Robert Kelly, Natalie Gold, Sol M. Crespo, Will Brill, Steven Maier, Matthew R. Staley, Matt Mancini, Emily Happe, Erin Wilhelmi, Joseph Reidy, Erika Rolfsrud, Elizabeth Goodman and Amy Irving. Written by Jonathan Bernstein and James Greer. Directed by Steven Soderbergh. At Assembly Row, Boston Common, Fenway, Seaport, South Bay and in the suburbs.